This invention relates generally to the field of fiber optics coupling devices and more particularly to a one to many fiber (1.times.N) single-mode holographic optical coupler.
As communications needs have expanded in modern times, the need for high speed, wide bandwidth communications networks has grown dramatically. Consequently, use of fiber optics communications networks have developed rapidly. Thus, there is a great need for optical fiber couplers, particularly for broad band single-mode couplers to permit interconnection to many users. One type of coupler which is highly useful for such systems is a 1.times.N coupler for distribution of the signal from one port to many ports (i.e., 1 fiber to many fibers). Multimode 1.times.N couplers exist in the prior art but are limited in bandwidth and mode structure and are thus unsuitable for many integrated optical systems. Thus, single-mode 1.times.N couplers are highly desirable. Demultiplexing 1.times.N couplers also exist in the prior art for demultiplexing N different wavelengths and channeling them to N output fibers, but such demultiplexers perform an entirely different function than a single-mode monochromatic 1.times.N fiber coupler.
Single-mode 1.times.N fiber couplers have been constructed using a tree of 3 dB fiber couplers. Alternatively, N.times.N star couplers can be utilizing as 1.times.N couplers by using only one input branch. Either one of these approaches, however, results in excessively complex fabrication and high cost. Another approach utilizes N consecutive taps on an input fiber with each tap having an appropriately adjusted coupling fraction to achieve a 1.times.N coupler. Integrated optics techniques have also been used to produce a 1.times.N coupler, but this approach limits N and introduces excess loss due to fiber-to-waveguide coupling losses and waveguide losses. A technique using lenses for coupling between input fibers and a bundle of output fibers has been suggested but results in low efficiency because of the large size differences between the core and cladding of typical single-mode fibers. Gratings have been suggested to generate multiple orders of diffraction from the input beam, but this approach requires lenses and accurate alignment. Thus, each of these approaches are limited in practice due to problems of cost, maximum size, efficiency, or difficulty of fabrication.
It is accordingly an object of the invention to provide a novel holographic 1.times.N single-mode fiber optics coupler which is efficient, cost effective, and easily fabricated.
It is another object of the invention to provide a novel holographic 1.times.N single-mode fiber optics coupler which can accommodate irregular fiber arrays and for which a whole monolithic array may be aligned at once, thereby simplifying fabrication.
It is yet another object of the invention to provide a novel holographic 1.times.N single-mode fiber optics coupler which is compact and has an overall efficiency approaching 50% regardless of the value of N.
Briefly, according to one embodiment of the invention, a holographic 1.times.N single-mode fiber optics coupler for dividing a single-mode fiber optics input into N single-mode outputs is provided. The apparatus includes input means for input of a single-mode fiber optics input signal, holographic means for holographically imaging the single-mode fiber optics input signal into an array of N single-mode optical signals, and output means for coupling the N optical signals to N output optical fibers.